Optical sensors (or transducers) for the measurement of various physical parameters such as pressure and temperature often rely on the transmission of strain to a sensing element (e.g., a fiber Bragg grating (FBG) or a fiber optic coil). One such parameter that is highly desirable to measure in oil/gas well applications is differential pressure. With a differential pressure measurement, parameters such as phase fraction, phase fraction flow rate, bulk fluid flow rate, and fluid density can be directly calculated. Such measurements and methods for calculating these and other parameters using optical flow meters are disclosed in the following U.S. patents and patent applications, which are incorporated herein by reference in their entireties: application Ser. No. 10/256,760, entitled, “Distributed Sound Speed Measurements for Multiphase Flow Measurement,” filed Sep. 27, 2002; application Ser. No. 10/186,382, entitled “Venturi Augmented Flow Meter,” filed Jun. 28, 2002; application Ser. No. 10/115,727, entitled “Flow Rate Measurement Using Unsteady Pressures,” filed Apr. 3, 2002; application Ser. No. 10/342,052, entitled “Phase Flow Measurement in Pipes Using a Density Meter,” filed Jan. 14, 2003; patent U.S. Pat. No. 6,354,147, entitled “Fluid Parameter Measurement in Pipes Using Acoustic Pressures,” issued Mar. 12, 2002.
Several optical sensors have been developed to measure differential pressure and which are useful in conjunction with the measuring schemes disclosed in the above-incorporated applications and patents. One example of such a sensor is found in U.S. Pat. No. 6,422,084, entitled “Bragg Grating Pressure Sensor,” issued Jul. 23, 2002, which is incorporated herein by reference. As disclosed in that patent, an optical sensor such as an FBG is housed in a housing into which a first pressure is ported. The diameter of the cladding around the FBG is increased, or the FBG is formed in a large diameter cladding, to form a relatively non-bendable sensing element whose optical properties (specifically, the Bragg reflection wavelength, λB, of the FBG) correlates to pressure. By affixing one end of the sensing element to a flexible wall (i.e., either a bellows or a diaphragm), and by exposing the outside of the housing to a second pressure, a differential pressure measurement is achieved. Additionally, by forming the sensing element in a “dog bone” structure, in which the FBG is located at a relatively smaller cladding diameter portion, the axial stress imparted to the FBG is increased, hence providing amplification of the strain and increasing the sensitivity of the sensing element.
However, this prior art differential pressure sensor may not be sensitive enough to measure small differential pressures of interest. For example, incorporated patent application Ser. No. 10/186,382 uses a venturi (i.e., restriction) contained within the pipe (e.g., an oil/gas well production pipe) to impart a differential pressure to the fluid flowing in the pipe. As noted above, measuring this differential pressure is useful in determining several parameters of the flowing fluid. However, this pressure differential might in a given application be quite small, and therefore difficult to resolve with needed accuracy when performing flow measurements.
It is known that optical sensors are sensitive to temperatures, a point which can be deleterious when it is desired that the sensor only measure pressure effects. For example, in an FBG based optical sensor, the FBG will expand or contract in response to increases or decreases in temperature in accordance with the coefficient of thermal expansion (CTE) of the (usually) quartz FBG element. Additionally, the index of refraction of the FBG (or other waveguide) will change with temperature. A FGB, as is known, is a periodic or aperiodic variation in the effective refractive index of an optical waveguide, similar to that described in U.S. Pat. Nos. 4,725,110 and 4,807,950 entitled “Method For Impressing Gratings Within Fiber Optics,” to Glenn et al. and U.S. Pat. No. 5,388,173, entitled “Method And Apparatus For Forming Aperiodic Gratings In Optical Fibers,” to Glenn, which are incorporated by reference in their entireties. Changes in temperature will cause the spacing, A, of the grating in the FBG to expand or contract, and will also affect the index of refraction, both of which affects the Bragg reflection wavelength, λB, of the sensor. (As is known and as is explained in the incorporated references, λB∝2neffΛ, where neff is the index of refraction of the core of the cane waveguide or optical fiber). These temperature-induced Bragg reflection wavelength shifts are preferably compensated for when the FGB is used to sense pressures.
The art would benefit from an optical differential pressure sensor capable of accurately resolving small differences in pressure, and which is minimally sensitive to temperature. Such a sensor is disclosed herein.